Various methods are known for the dry desulfurizing of flue gases of different origin. For example, such a method is proposed in the German Offenlegungsschrift 31 17 601 for dry desulfurizing and removing other impurities in flue gases from coal-fired power plants or garbage incinerators, for which dry calcium hydroxide, for example, is used as absorbent (additive). According to this method, the additive is added directly into the crude gas segment, optionally before a reactor lying in this segment and supplied to a downstream filtering installation. The solid throughput obtained here passes through one or several cyclone precipitators and can then be discharged from the associated equipment separated from the purified off-gas.
Aside from this dry purification, "wet" methods are also known. In the German Offenlegungsschrift 33 34 106, a wet method is disclosed, for which the alkaline calcium compounds are sprayed in liquid form as absorbent (additive) into the crude gas. It is a disadvantage of this method that the additive, added several times in the stoichiometric amount based on the pollutants, cannot be reacted intensively enough and, particularly in the case of fluctuations, an optimum adaptation of the dosage is hardly possible.
A better utilization of the additives and higher degrees of flue gas desulfurization are said to be attained pursuant to the German Offenlegungsschrift 40 04 866 for boilers with coal dust firings and grate firings in that suspensions of additives that bind sulfur dioxide are introduced at an elevated pressure over known nozzle systems into the free spaces carrying the flue gas.
One of the disadvantages of this method is the fact that the temperature of the suspension must not exceed or fall below certain values after the pressure is increased; for example, the temperature of the suspension must be adjusted by the indirect transfer of heat so that it is at least 10.degree. K. above the temperature of the steam dew point temperature of the flue gas that is to be purified, but no more than 3.degree. K. below the boiling point of the suspension at the pressure before the nozzles.
It is furthermore known that the use of sulfur-containing bituminous coal as a fuel is meaningful only if the flue gas is desulfurized at the same time. However, the costs of the chemicals and equipment required for this at least partially cancel out the price advantage of the coal. Whether and to what extent this fuel can remain competitive depends greatly on the subsidiary costs, which arise from the desulfurizing of the flue gases. The competitiveness of this coal is improved by reducing these subsidiary costs.